The present invention relates generally to transfer cases for use in four-wheel drive vehicles. More particularly, the present invention relates to a transfer case having a two-speed gear reduction unit and a synchronized range shift mechanism for permitting on-the-fly shifting between high-range and low-range drive modes.
As is now conventional, many light-duty and sport-utility vehicles are equipped with a transfer case for transmitting drive torque to all four of the wheels, thereby establishing a four-wheel drive mode of operation. To accommodate differing road surfaces and conditions, many transfer cases are equipped with a gear reduction unit which can be selectively shifted to permit the vehicle operator to choose between a four-wheel high-range (i.e., direct ratio) drive mode and a four-wheel low-range (i.e., reduced ratio) drive mode. In many instances, the four-wheel drive vehicle must be stopped before the transfer case can be shifted between its four-wheel high-range and low-range drive modes. Unfortunately, the need to stop the vehicle prior to shifting between the available four-wheel high-range and low-range drive modes is inconvenient, particularly upon encountering road conditions or surface terrains where continuation of the vehicle's rolling momentum would assist in overcoming the conditions encountered. To alleviate a portion of this inconvenience, some gear reduction units have been designed which permit the vehicle operator to shift without stopping the vehicle (i.e., "on-the-fly") from the four-wheel low-range drive mode into the four-wheel high-range drive mode. For example, U.S. Pat. No. 5,054,335 discloses a transfer case equipped with a synchronized range shift arrangement for "on-the-fly" shifting of a layshaft-type gear reduction unit. Alternatively, commonly-owned U.S. Pat. No. 5,346,442 discloses a transfer case having a synchronized range shift arrangement for "on-the-fly" shifting of a planetary-type gear reduction unit. Finally, U.S. Pat. No. 4,569,252 discloses a planetary-type gear reduction unit which permits synchronized shifting into and out of the high-range drive mode and the low-range drive mode.
In addition to the gear reduction unit, many transfer cases are also equipped with a mode shift mechanism which permits the vehicle operator to selectively shift between a two-wheel drive mode wherein only the primary (i.e., rear) driveline is driven and a "part-time" four-wheel drive mode wherein the secondary (i.e., front) driveline is rigidly coupled for rotation with the primary driveline. Reference may be made to commonly-owned U.S. Pat. No. 4,770,280 for disclosure of an exemplary part-time transfer case equipped with a gear reduction unit and a synchronized mode shift mechanism. In view of increased consumer popularity in four-wheel drive vehicles for everyday use, the mode shift mechanism in some two-speed transfer cases is replaced with a multi-plate clutch assembly which is operable for transmitting drive torque automatically (i.e., on-demand) to the secondary driveline, without any input or action on the part of the vehicle operator, when traction is lost at the primary wheels. Reference may be made to commonly-owned U.S. Pat. No. 5,363,938 for disclosure of an exemplary two-speed transfer case equipped with a clutch assembly interactively associated with an electronic control system and a sensor arrangement. While such prior art arrangements provide a compact construction, there is a continuing need to develop low cost, simplified alternatives which meet modern requirements for low noise and weight.